Intriguing Temperatures on Enceladus

by Paul Gilster on March 28, 2008

Cassini’s recent pass through the plumes of Enceladus resulted in a number of intriguing finds, perhaps the most interesting of which is the temperature along the ‘tiger stripes.’ These are the fissures from which Enceladus’ famous geysers erupt. Cassini’s Composite Infrared Spectrometer found them to be warm along almost their entire length, reaching no less than minus 93 degrees Celsius (-135 F). The warmest regions correspond to two visible geyser locations. The contrast in temperatures is striking: The differential between these regions and other areas on Enceladus is a whopping 93 degrees Celsius (200 F).

This heat map gives a sense of what we’re dealing with. The brightest fracture, known as Damascus Sulcus and visible at lower left in the image, shows the highest temperatures. In this image, the false color infrared data are superimposed on a grayscale mosaic of the south pole that dates back to the summer of 2005. The map was made at a distance of between 14,000 and 32,000 kilometers starting sixteen minutes after closest approach as the spacecraft receded.

Image: Heat radiating from the entire length of 150 kilometer (95 mile)-long fractures is seen in this best-yet heat map of the active south polar region of Saturn’s ice moon Enceladus. The warmest parts of the fractures tend to lie on locations of the plume jets identified in earlier images, shown in the annotated version with yellow stars. The measurements were obtained by the Cassini spacecraft’s Composite Infrared Spectrometer from the spacecraft’s close flyby of the moon on March 12, 2008. Credit: NASA/JPL/Space Science Institute.

John Spencer (Southwest Research Institute), a scientist on the spacecraft’s Composite Infrared Spectrometer team, notes the significance of the new findings:

“These spectacular new data will really help us understand what powers the geysers. The surprisingly high temperatures make it more likely that there’s liquid water not far below the surface.”

Surprising indeed, and encouraging, for we’re looking at an Enceladus with unusual warmth and possible liquid water. Moreover, Cassini’s pass through the plumes led to the detection of organic chemicals, a mix reminiscent of cometary materials. The density of volatile gases, water vapor, carbon dioxide and carbon monoxide, along with organic materials both simple and complex, was twenty times what had been expected.

Image: Enceladus’ plume was found to have a comet-like chemistry by Cassini’s Ion and Neutral Mass Spectrometer during its fly-through of the plume on Mar. 12, 2008. Water vapor, methane, carbon monoxide, carbon dioxide, simple organics and complex organics were identified in the plume. The graph shows the chemical constituents in percentage of abundance found in comets compared to those found in Enceladus’ plume. Credit: NASA/JPL/SwRI.

In a NASA feature on the Saturnian moon, Chris McKay (NASA Ames) discusses microbial ecosystems that could be models for life on Enceladus today:

There are three such ecosystems found on Earth that would conceivably be a basis for life on Enceladus. Two are based on methanogens, which belong to an ancient group related to bacteria, called the archaea — the rugged survivalists of bacteria that thrive in harsh environments without oxygen. Deep volcanic rocks along the Columbia River and in Idaho Falls host two of these ecosystems, which pull their energy from the chemical interaction of different rocks. The third ecosystem is powered by the energy produced in the radioactive decay in rocks, and was found deep below the surface in a mine in South Africa.

But if life may be feasible on Enceladus, how would it begin? Organic chemicals, notes McKay, were part of the raw materials from which the moons of Saturn formed. Other ingredients could have arrived on comets or interplanetary dust. Add a heating mechanism — tidal heating is one candidate, assuming an earlier oblong orbit, or earlier tidal relationships with another moon — and you could eventually produce a subsurface aquifer rich in organic materials, what McKay calls a ‘prebiotic soup.’

Every time we look at Enceladus the excitement seems to build, and the next flyby isn’t until August! Another passage through the plumes would certainly add to our data, while it’s clear that Enceladus is also moving up on the interest scale in terms of future missions. A lander that could take plume samples and explore the area near the fissures would be optimal. We can only guess when budget realities might make such a mission possible, but until then the data harvest from our existing orbiter is splendid, with analysis of the organic compounds in the plume and the sources of Enceladus’ energy likely to occupy us for years to come.

So it seems the water model is more likely, however the fact that the activity is concentrated in a single region may suggest that there isn’t a global ocean. It is also not entirely clear that the liquid has been there for the entire history of the solar system – the known energy sources don’t seem to add up. Maybe this is a transient episode of localised heating (perhaps from an impact), or perhaps the last vestiges of a former ocean that is now almost entirely frozen.

A further question, following up on andy’s point, is how long does it take for life to emerge given the necessary ingredients? It may not have been necessary for liquid to be available within Enceladus since the formation of the Solar System, but would a hundred million years suffice? Ten million? We have much to learn not only about how life emerges but within what time frames.

Chronicling a bold voyage of discovery — the Cassini/Huygens mission
to Saturn and its enigmatic moon Titan — NOVA’s “Voyage to the
Mystery Moon” delivers striking images of these fascinating planetary
bodies nearly a billion miles from Earth. Saturn’s broad rings hold
myriad mysteries, and Titan, whose soupy atmosphere is similar to the
one that enshrouded our planet billions of years ago, may hold clues
to the origins of life.

Ganymede has a diameter of 3,280 miles, Titan has a diameter of 3,200 miles, Triton has a diameter of 2,361 miles, Europa has an equatorial radius of 1,565 km, Io has an equatorial radius of 1830 x 1818.7 x 1815.3 km, Callisto has diameter of over 4,800 km (2,985 miles), the Earth’s Moon has a diameter of 2,160 miles.

In addition, there are numerous solar system moons with a diameter between 300 km and 2500 km as well as smaller moons.

Altogether, there are 139 moons within the solar system. This is a lot of territory for both unmanned and manned space craft to explore. Even before we venture out into the Kuiper Belt, Oort Cloud, and then on to our stellar neighbors and beyond in terms of manned interstellar space craft, we have much territory right here within the solar system to explore.

For me, part of the fun of taking a series of ever more ambitious, ever more elaborate, and ever more long duration road trip vacations here on Earth is simply just going out to fuel the car up just before one leaves on the first and least extravagent road trip to the nearest destination. I seem to have the same sense of excitement as I see the ISS nearing completion, the CEV being developed to send humans back to the Moon by 2020, the plan by 2024 to have a permanent manned outpost on the Moon, rudementary plans to send human to Mars a decade or decade and a half later, and then to “worlds beyond’ according to President Bush’s enourmously popular speech among us space heads made a few year ago.

Explanation: Could life exist beneath Enceladus? A recent flyby of Saturn’s icy moon has bolstered this fascinating idea. Two years ago, images from the robotic Cassini spacecraft orbiting Saturn led astronomers to the undeniable conclusion that Saturn’s moon Enceladus was spewing fountains of gas and ice crystals through cracks in its surface dubbed tiger stripes.

Last month, Cassini dove through some of these plumes and determined that they contained water vapor laced with small amounts of methane as well as simple and complex organic molecules.

Surprisingly, the plumes of Enceladus appear similar in make-up to many comets. What’s more, the temperature and density of the plumes indicate they might have originated from a warmer source — possibly a liquid source — beneath the surface. A liquid water sea containing organic molecules is a good place to look for life.

Pictured above is a vertically exaggerated close-up of some long, venting tiger stripes. The computer composite was generated from images and shadows taken during the recent Cassini flyby. Nine more flybys of Enceladus by Cassini are planned.

Enceladus, is like that gift under the tree at Christmas, total excitement, as you never know what’s inside till you open it up. What is more surprising is why the color of the tiger stripes on Enceladus have not been better examined and explained. We are after all looking at a moon that reflects almost 100% light off it’s surface. So if you add up all the recent finds about these strange tiger stripe features, the warmth, the geysers, organics signatures, and so on. Does this lead to a conclusion that the color of the stripes come from organic material that is alive? We may be looking right at life in these photos. I have yet to hear anything about why theses stripes are green, is what I guess I’m getting at.

The operations of the Cassini spacecraft, part of the
international NASA/ESA/ASI Cassini-Huygens mission, have
been extended by NASA by two years. The historic mission’s
stunning discoveries and images have revolutionised our
knowledge of Saturn and its moons.

Abstract: The detection of outgassing water vapor from Enceladus is one of the great breakthroughs of the Cassini mission. The fate of this water once ionized has been widely studied; here we investigate the effects of purely neutral-neutral interactions within the Enceladus torus. We find that, thanks in part to the polar nature of the water molecule, a cold (~180 K) neutral torus would undergo rapid viscous heating and spread to the extent of the observed hydroxyl cloud, before plasma effects become important.

We investigate the physics behind the spreading of the torus, paying particular attention to the competition between heating and rotational line cooling. A steady-state torus model is constructed, and it is demonstrated that the torus will be observable in the millimeter band with the upcoming Herschel satellite. The relative strength of rotational lines could be used to distinguish between physical models for the neutral cloud.

The tide may be changing for the ocean suspected under the icy shell of Enceladus. Recent research has shown that this small moon of Saturn does not produce enough heat in its present configuration to keep water from freezing down to its core.

“There is no possible combination of parameters that allow for a thermally stable ocean,” said James Roberts of the University of California, Santa Cruz.

Roberts and his colleague Francis Nimmo, also from UCSC, calculated the tidal heating expected inside Enceladus from the uneven tugging of nearby Saturn.

In all the models studied, the moon could not sustain an ocean for more than around 30 million years.

This could mean the ocean froze up long ago. Or perhaps more likely, Enceladus generated a greater amount of heat at some point during the past 30 million years by being in a more eccentric orbit than it is in now, the authors claim in a recent article for the journal Icarus.

Charter

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last nine years, this site has coordinated its efforts with the Tau Zero Foundation, and now serves as the Foundation's news forum. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

On Comments

Centauri Dreams publishes selected comments on the articles under discussion here. The primary criterion is that comments contribute meaningfully to the debate. Among other criteria for selection: Comments must be on topic, directly related to the post in question, must use appropriate language and must not be abusive to others. Civility counts. In addition, a valid email address is required for a comment to be considered. Centauri Dreams is emphatically not a soapbox for political or religious views submitted by individuals or organizations. A fuller statement of the policy can be viewed on the Administrative page.